The present invention relates, in general, to a method and apparatus for tracking and guiding the drilling of a borehole, and more particularly to tracking a borehole being drilled generally horizontally under an obstacle such as a river, stream, lake, swampy area, or the like where access to the ground above the borehole is difficult or perhaps even restricted.
Various well-known drilling techniques have been used in the placement of underground transmission lines, communication lines, pipelines or the like through or beneath obstacles of various types. In order to traverse the obstacle, the borehole must be tunnelled underneath the obstacle from an above-ground entry point to a desired exit point, with the borehole then serving to receive a casing, for example, for use as a pipeline or for receiving cables for use as power transmission lines, communication lines, or the like. In the drilling of such boreholes, it is important to maintain them on a carefully controlled track, for often the borehole must remain within a right of way as it passes under the obstacle, and its entry and exit points on opposite sides of the obstacle must often be within precisely defined areas.
Prior systems for providing guidance in the drilling of boreholes have presented problems to the user, since they require access to the earth's surface above the location of the borehole to permit placement of grids or other guidance systems on the surface of the earth, above the paths to be followed by the borehole. Often, however, access to this region is not available. Furthermore, the placement of guide cables of this kind can be extremely time consuming, and thus expensive, and accordingly an improved method of guidance has been actively sought in the art.